This invention relates to an extraction agent employed for solvent extraction of palladium and platinum and a process of recovering these precious metals by extracting and back-extracting same by means of the extraction agent.
A variety of substances have been conventionally reported which are employed for recovering palladium and platinum as a solvent extraction agent.
Trioctyl phosphine oxide (TOPO), methyl isobutyl ketone (MIBI), tributyl phosphate (TBP) and trioctylamine are known as an extraction agent of palladium and platinum from a hydrochloric acid solution, and diethyldithiocarbamic acid is known as a chelate extraction agent.
These agents possess such disadvantages as poor selectivity of recovering only precious metals, insufficient extraction efficiency, slow extraction speed and difficulty of back-extraction after extraction to an organic phase.
In accordance with a concept of "hard acid, soft acid and hard base, soft base" proposed by Mr. R. G. Pearson (reference is made to J. Amer. Chem. Soc. 3533 (1963)), palladium and platinum are typically classified in soft acids.
Accordingly, in order to selectively extract these metals, a sulfur compound which is classified in soft bases is preferably employed as an extraction agent.
Dihexyl sulfide and dioctyl sulfides which are synthetically prepared and petroleum sulfides contained in crude oil are known as the above sulfur compound, and are employed in practical processes.
In case of employing these extraction agents, however, the extraction of palladium and platinum requires a long period of time such that several hours are required to attain the equilibrium in case of the palladium and extraction of tetravalent platinum can be seldom performed in a period of practicable hours.